A liquid crystal display indispensably requires polarizing elements disposed on both sides of a liquid crystal cell because of an image forming method adopted therein and generally polarizing plates are adhered. Besides, on a liquid crystal panel, various kinds of optical elements have been used in addition to a polarizing plate in order to improve a display quality of a display. For example, there have been used a retardation plate for coloration prevention, a viewing angle increasing film for improving a viewing angle of a liquid crystal display and a brightness enhancement film for raising a contrast of a display. The films each are collectively referred to an optical film.
A pressure-sensitive adhesive is usually employed in adhering an optical film described above to a liquid crystal cell. An optical film and a liquid crystal cell or optical films are usually adhered to each other using a pressure-sensitive adhesive therebetween in order to reduce a light loss. In such cases, a pressure-sensitive adhesive optical film in which a pressure-sensitive adhesive is provided in advance on one surface of an optical film as a pressure-sensitive adhesive layer is generally used because of a merit such as that no necessity arises for a drying step of fix the optical film.
Necessary features required for the pressure-sensitive adhesive are that (1) even when the process of bonding an optical film to a liquid crystal panel surface is performed in a wrong position or accompanied by a foreign substance trapped on the bonded surface, the optical film can be separated from the liquid crystal panel surface and bonded again (reworked); (2) the pressure-sensitive adhesive has stress relaxation properties in order to prevent optical unevenness which would otherwise be caused by a dimensional change of the optical film; (3) defects responsible for the pressure-sensitive adhesive do not occur in an endurance test by heating, humidifying or the like generally performed as an accelerated environment test; and so on.
However, there is a problem in which the release forces of the pressure-sensitive adhesive increases with time so that it can be very difficult to separate the optical film from the adherend at the time of reworking or at the time of separating and collecting the liquid crystal panel. Particularly when the pressure-sensitive adhesive used has improved durability, the separation of the optical film should be difficult.
In order to solve the above problem with release property, it is proposed to bond an optical film to glass through a weak pressure-sensitive adhesive layer with low adhesive strength (see Patent Literature 1 below). However, this technique has a problem with the durability of the pressure-sensitive adhesive layer. There is also proposed a separation method in which when a polarizing plate is separated from the surface of a liquid crystal cell, the temperature of the polarizing plate is lowered (see Patent Literature 2 below). However, this method has a problem in which operational efficiency is lowered. There is also proposed a method that includes cutting a polarizing plate while leaving it attached to the surface of a liquid crystal panel, attaching the cut pieces of the polarizing plate to a pressure-sensitive adhesive tape and separating them from the panel (see Patent Literature 3 below). However, this method has a problem in which the liquid crystal panel can be damaged.
In contrast, the optical film bonded to the liquid crystal panel with the pressure-sensitive adhesive interposed therebetween can easily release at high humidity. In order to improve durability in a humidified environment and the like, therefore, a coupling agent is generally added to an optical pressure-sensitive adhesive (see Patent Literatures 4 and 5 below). Such a pressure-sensitive adhesive can prevent the separation of the optical film in a humidified environment but accordingly has higher adhesive strength to make releasing difficult at the time of separation and collection.    Patent Literature 1: JP-A No. 2004-347655    Patent Literature 2: JP-A No. 10-73795    Patent Literature 3: JP-A No. 2002-159955    Patent Literature 4: JP-A No. 57-195208    Patent Literature 5: JP-A No. 08-104855